This invention relates generally to gasifiers and, more particularly, to an automated system for monitoring the condition of and for agitating the coal bed inside a coal gasifier.
Two-stage, fixed-bed, coal gasifiers have been in existence for many years, having been developed and employed mainly in Europe and South Africa. In operation, coal is fed into the upper or distillation zone of the gasifier, while a mixture of steam-and-air is introduced into the bottom. A small amount of coal is completely oxidized to provide the heat required to gasify carbon in the lower zone and distillation of the feed coal volatile matter in the upper zone. The gases thus produced are collected and used as a fuel in burners designed for this purpose.
The condition of the lower zone coal bed inside the gasifier has to be checked periodically in accordance with a planned, maintenance schedule. Generally, this checking procedure is accomplished by manually inserting a steel pokerod into one of several pokeholes, or ports, which are spaced around the perimeter of the gasifier and which orient the pokerod, and pushing the pokerod to penetrate the layer of coal, and to stop at the center of a grate positioned at the bottom of the gasifier. The insertion of the pokerod performs two important functions: (1) the detection of clinkers, or coal which has been fused together, in the fire zone, which is achieved by the operator feeling obstructions to the passage of the pokerod being inserted into the coal bed; and (2) the measurement of the depth of the fire zone, the area of the most intense burning of the coal, which is accomplished by visually observing the discoloration of the pokerod after it has been left in place for a couple of minutes, and then withdrawn.
Because of the frequency of the required poking operation, and the number of pokeholes which must be checked, this procedure for checking the condition of the coal bed usually requires from one to three operators per shift, depending upon the number of gasifiers at the plant. Where the labor cost is relatively inexpensive, this procedure presents no problem. However, in high labor cost markets, such as the United States, for example, this poking operation becomes a costly operating expense. In addition, the determination of incipient clinker formation and bed temperature is achieved in an unscientific manner and depends upon the human reaction to the poking force and pokerod color. A further problem is that these characteristics are usually transmitted verbally to the person actually in charge of making the changes necessary.
When coal is heated in the absence of oxygen, the coal structure expands, or swells and the free swelling index is used as a measure of the amount of swelling experienced by the coal. The use of coal with a free swelling index greater than 2.7 in this type of fixed-bed gasifier is not now possible since the coal will swell and plug the distillation zone of the gasifier where the coal is heated to the 700.degree.-800.degree. F. temperature range. By agitating or poking the coal bed, the gases causing the swelling are released from the coal structure to reduce excessive swelling, and thus permit the use in a gasifier of coal having a free swell index greater than 2.7.